1. Field of the Invention
The present invention relates to the production of a novel REBaCuO oxide superconductor or superconducting oxide and, more particularly, to the fabrication of a superconducting oxide having large magnetic levitation force, which is intended for use with fly wheels, magnetic bearings; delivery systems, etc., that harness magnetic levitation.
2. Prior Art
In recent years, REBaCuO superconducting oxides have started to be used for application to fly wheels, etc., by magnetic levitation. These superconductors, for instance, have been produced by the melt powder melt growth (MPMG) technique--see H. Fujimoto and co-workers, "Proc. of ISS '89", Sprinter-Verlag, p. 285, 1990.
Set out below is one typical example of superconductor production by this technique. First, the powder feed, e.g., Y.sub.2 O.sub.3, BaCO.sub.3 and CuO, are mixed together at a predetermined ratio. The mixture may have been calcined and pulverized. Further, the powders are heated to a temperature at which RE.sub.2 O.sub.3 and liquid phases coexist, e.g., 1,400.degree. C. for partial melting (M). Subsequently, the melt is cooled down for solidification. This is followed by pulverization (P), mixing and shaping. The shape is then heated to a temperature at which an RE.sub.2 BaCuO.sub.5 phase (hereinafter referred simply to as the 211 phase) and a liquid phase coexist, e.g., 1,100.degree. C. for partial melting (M). After this, the partial melt is cooled down to a temperature at which a superconducting REBa.sub.2 Cu.sub.3 O.sub.d phase (hereinafter simply called the 123 phase) forms, from which it is slowly cooled at a given rate, e.g., 1.degree. C./h, thereby forming and growing (G) the 123 phase for superconductor production.
A superconductor fabricated with the use of this technique shows a high critical current density. In order to increase the magnetic levitation force of this superconductor, however, it is required not only to elevate the critical current density but to increase superconducting crystal size as well--see M. Murakami and co-workers, "Japanese Journal of Applied Physics", Vol. 29, No. 11, p. 1991, 1990. For increasing superconducting crystal size, crystal formation and growth must be placed under control. So far, however, this control could not have be done; that is, some considerable difficulty has been involved in producing a superconductor composed of large superconducting crystals and having increased magnetic levitation force.
Another approach for superconductor production is the quench and melt growth (QMG) technique--see M. Murakami and co-workers, "Japanese Journal of Applied Physics", Vol. 28, No. 7, p. 1189, 1989. Set out below is one typical example of superconductor production by this technique. First, the powder feed, e.g., Y.sub.2 O.sub.3, BaCO.sub.3 and CuO, are mixed together at a predetermined ratio. Then, the mixture is heated to a temperature at which RE.sub.2 O.sub.3 and liquid phases coexist, e.g., 1,400.degree. C. for the partial melting of the raw material powders, followed by quenching (Q) or rapid cooling for solidification. In this rapidly solidified structure, the RE.sub.2 O.sub.3 phase is dispersed throughout the solidified liquid phase. The rapidly solidified structure is then heated to a temperature at which the 211 and liquid phases coexist, e.g., 1,100.degree. C. for partial melting (M). After this, the melt is cooled down to a temperature at which the 123 phase forms, whence it is slowly cooled at a given rate, e.g., 1.degree. C./hour, thereby forming and growing (G) the 123 phase for superconductor production. This technique may be said to be a process alternative to the above MPMG technique, from which the steps of pulverizing, mixing and shaping the solidified body are removed. A superconductor fabricated by this technique, too, shows a locally high critical current density.
However, the thickness of the rapidly solidified body mentioned above is at most about 5 mm in a single, simple quenching operation; any thick superconductor could not be obtained by the QMG technique. To ward off this disadvantage, the so-called multi-quenching technique has been developed--see M. Morita and co-workers, "Proc of ISS '90," Springer-Verlag, p.733, 1991. This technique may prima facie enable some thick superconductor to be fabricated, but leaves the following problems intact.
(1) Mechanical processing is needed for forming the rapidly solidified (quenched) body into any desired shape. However, it is very difficult to obtain any desired shape by mechanical processing, because the rapidly solidified body is so brittle that mechanical processing is difficult for shaping.
(2) In carrying out the multi-quenching technique, the already quenched, solidified body is unavoidably affected by the thermal history of the partial melt that is to be quenched later.
(3) In order to fabricate a thick solidified body by multi-quenching, it is required to carry out quenching several times, and this is labor-consuming.
(4) The RE.sub.2 O.sub.3 phase dispersed in the quenched, solidified body is relatively large in size, and so makes it difficult to obtain a desired superconducting structure--see Fujimoto and co-workers, "Gryogenic Engineering", Vol. 25 No. 2, p. 77, 1990.
The MPMG technique, because of being designed to carry out shaping after the pulverization and mixing of the solidified body, has some merits, mentioned just below, and so can be taken as being a very excellent method.
(1) The mixture can be shaped in any desired configuration.
(2) It is possible to add useful additives such as silver oxide to the solidified powders in the course of mixing them.
(3) The solidification and mixing of the solidified body permits the RE.sub.2 O.sub.3 phase dispersed therein to be concurrently pulverized and mixed, giving a fine and uniform RE.sub.2 O.sub.3 phase--see Fujimoto and co-workers, "Gryogenic Engineering", Vol. 25, No. 2, p. 77, 1990.
According to some report, one recently developed technique for increasing superconducting crystal size has well succeeded in achieving crystal size increases by seeding a SmBa.sub.2 Cu.sub.3 O.sub.d single crystal--that is produced by the QMG technique and then cleaved in precursor to which a Yb--Y composition gradient is imparted with a Yb.sub.1-x Y.sub.x Ba.sub.2 Cu.sub.3 O.sub.d composition--at around 1,030.degree. C. (in the cooling process in the MG steps of the QMG technique or anytime from the partial melting to the initiation of the slow cooling for forming and growing the superconducting phase). For this, see M. Morita and co-workers, "Proc. of ISS '90", Springer-Verlag, p. 733, 1991. A patent specification directed to this method has already been published for public inspection (under No. WO91/19029). However, this method is unfit for mass production, because much time and labor are needed for fabricating the precursor having a composition gradient, producing the above seed single crystal, and placing it on the precursor.
According to some reports on the MPMG technique, on the other hand, when silver or silver oxide is added at the time of the above pulverization of the solidified powders, cracks in the produced superconductor are reduced well-enough to improve the magnetic levitation force--see M. Murakami and co-workers, "Japanese Journal of Applied Physics", Vol. 29, No. 11, p. 1991, 1990. Further, according to another report, when platinum or a platinum compound is added at the time of the pulverization of the solidified powders, a critical current density of the produced superconductor is improved (Japanese patent application No. 68627/1991). Even when these additives are used, crystal formation and growth must be placed under control, as is the case with the absence of any additive, thereby achieving increases in superconducting crystal size and magnetic levitation force.
In view of the problems mentioned above, a primary object of the invention is to provide a method for producing a superconducting oxide having large magnetic levitation force, which enables the superconducting crystals obtained by the MPMG technique to be increased in their size and so improved in terms of their magnetic levitation force.